Planning on doing this ride some time soon.
I haven't looked at a map to verify yet, but try this:
go to byCycle.org 's TripPlanner page for Portland:
Copy and Paste this into the "To:" line under "Find a Route"
This is the coordinates for Timberline Lodge.
Add your address for a starting address and wait for the Trip Planner to do it's thing.
Hope this helps!
I'll post a report if I do the ride in the next couple of weeks.